Add history graph API

This new API allows the commit history to be displayed as a text-based
graphical representation.

Signed-off-by: Adam Simpkins <adam@adamsimpkins.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/technical/api-history-graph.txt

@@ -0,0 +1,176 @@
+history graph API
+=================
+
+The graph API is used to draw a text-based representation of the commit
+history.  The API generates the graph in a line-by-line fashion.
+
+Functions
+---------
+
+Core functions:
+
+* `graph_init()` creates a new `struct git_graph`
+
+* `graph_release()` destroys a `struct git_graph`, and frees the memory
+  associated with it.
+
+* `graph_update()` moves the graph to a new commit.
+
+* `graph_next_line()` outputs the next line of the graph into a strbuf.  It
+  does not add a terminating newline.
+
+* `graph_padding_line()` outputs a line of vertical padding in the graph.  It
+  is similar to `graph_next_line()`, but is guaranteed to never print the line
+  containing the current commit.  Where `graph_next_line()` would print the
+  commit line next, `graph_padding_line()` prints a line that simply extends
+  all branch lines downwards one row, leaving their positions unchanged.
+
+* `graph_is_commit_finished()` determines if the graph has output all lines
+  necessary for the current commit.  If `graph_update()` is called before all
+  lines for the current commit have been printed, the next call to
+  `graph_next_line()` will output an ellipsis, to indicate that a portion of
+  the graph was omitted.
+
+The following utility functions are wrappers around `graph_next_line()` and
+`graph_is_commit_finished()`.  They always print the output to stdout.
+They can all be called with a NULL graph argument, in which case no graph
+output will be printed.
+
+* `graph_show_commit()` calls `graph_next_line()` until it returns non-zero.
+  This prints all graph lines up to, and including, the line containing this
+  commit.  Output is printed to stdout.  The last line printed does not contain
+  a terminating newline.  This should not be called if the commit line has
+  already been printed, or it will loop forever.
+
+* `graph_show_oneline()` calls `graph_next_line()` and prints the result to
+  stdout.  The line printed does not contain a terminating newline.
+
+* `graph_show_padding()` calls `graph_padding_line()` and prints the result to
+  stdout.  The line printed does not contain a terminating newline.
+
+* `graph_show_remainder()` calls `graph_next_line()` until
+  `graph_is_commit_finished()` returns non-zero.  Output is printed to stdout.
+  The last line printed does not contain a terminating newline.  Returns 1 if
+  output was printed, and 0 if no output was necessary.
+
+* `graph_show_strbuf()` prints the specified strbuf to stdout, prefixing all
+  lines but the first with a graph line.  The caller is responsible for
+  ensuring graph output for the first line has already been printed to stdout.
+  (This can be done with `graph_show_commit()` or `graph_show_oneline()`.)  If
+  a NULL graph is supplied, the strbuf is printed as-is.
+
+* `graph_show_commit_msg()` is similar to `graph_show_strbuf()`, but it also
+  prints the remainder of the graph, if more lines are needed after the strbuf
+  ends.  It is better than directly calling `graph_show_strbuf()` followed by
+  `graph_show_remainder()` since it properly handles buffers that do not end in
+  a terminating newline.  The output printed by `graph_show_commit_msg()` will
+  end in a newline if and only if the strbuf ends in a newline.
+
+Data structure
+--------------
+`struct git_graph` is an opaque data type used to store the current graph
+state.
+
+Calling sequence
+----------------
+
+* Create a `struct git_graph` by calling `graph_init()`.
+
+* Use the revision walking API to walk through a group of contiguous commits.
+
+* For each commit traversed, call `graph_update()` to move the graph to the
+  next commit.  Once `graph_update()` has been called, call `graph_next_line()`
+  repeatedly, until `graph_is_commit_finished()` returns non-zero.  Each call
+  to `graph_next_line()` will output a single line of the graph.  The resulting
+  lines will not contain any newlines.  `graph_next_line()` returns 1 if the
+  resulting line contains the current commit, or 0 if this is merely a line
+  needed to adjust the graph before or after the current commit.  This return
+  value can be used to determine where to print the commit summary information
+  alongside the graph output.
+
+Limitations
+-----------
+
+* `graph_update()` must be called with commits in topological order.  It should
+  not be called on a commit if it has already been invoked with an ancestor of
+  that commit, or the graph output will be incorrect.
+
+* `graph_update()` must be called on a contiguous group of commits.  If
+  `graph_update()` is called on a particular commit, it should later be called
+  on all parents of that commit.  Parents must not be skipped, or the graph
+  output will appear incorrect.
++
+`graph_update()` may be used on a pruned set of commits only if the parent list
+has been rewritten so as to include only ancestors from the pruned set.
+
+* The graph API does not currently support reverse commit ordering.  In
+  order to implement reverse ordering, the graphing API needs an
+  (efficient) mechanism to find the children of a commit.
+
+Sample usage
+------------
+
+------------
+struct commit *commit;
+struct git_graph *graph = graph_init();
+
+while ((commit = get_revision(opts)) != NULL) {
+	graph_update(graph, commit);
+	while (!graph_is_commit_finished(graph))
+	{
+		struct strbuf sb;
+		int is_commit_line;
+
+		strbuf_init(&sb, 0);
+		is_commit_line = graph_next_line(graph, &sb);
+		fputs(sb.buf, stdout);
+
+		if (is_commit_line)
+			log_tree_commit(opts, commit);
+		else
+			putchar(opts->diffopt.line_termination);
+	}
+}
+
+graph_release(graph);
+------------
+
+Sample output
+-------------
+
+The following is an example of the output from the graph API.  This output does
+not include any commit summary information--callers are responsible for
+outputting that information, if desired.
+
+------------
+*
+*
+M
+|\
+* |
+| | *
+| \ \
+|  \ \
+M-. \ \
+|\ \ \ \
+| | * | |
+| | | | | *
+| | | | | *
+| | | | | M
+| | | | | |\
+| | | | | | *
+| * | | | | |
+| | | | | M  \
+| | | | | |\  |
+| | | | * | | |
+| | | | * | | |
+* | | | | | | |
+| |/ / / / / /
+|/| / / / / /
+* | | | | | |
+|/ / / / / /
+* | | | | |
+| | | | | *
+| | | | |/
+| | | | *
+------------