blame: add --line-porcelain output format

This is just like --porcelain, except that we always output
the commit information for each line, not just the first
time it is referenced. This can make quick and dirty scripts
much easier to write; see the example added to the blame
documentation.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/blame-options.txt

@@ -52,6 +52,11 @@ of lines before or after the line given by <start>.
 --porcelain::
 	Show in a format designed for machine consumption.
 
+--line-porcelain::
+	Show the porcelain format, but output commit information for
+	each line, not just the first time a commit is referenced.
+	Implies --porcelain.
+
 --incremental::
 	Show the result incrementally in a format designed for
 	machine consumption.

Documentation/git-blame.txt

@@ -105,6 +105,19 @@ The contents of the actual line is output after the above
 header, prefixed by a TAB. This is to allow adding more
 header elements later.
 
+The porcelain format generally suppresses commit information that has
+already been seen. For example, two lines that are blamed to the same
+commit will both be shown, but the details for that commit will be shown
+only once. This is more efficient, but may require more state be kept by
+the reader. The `--line-porcelain` option can be used to output full
+commit information for each line, allowing simpler (but less efficient)
+usage like:
+
+	# count the number of lines attributed to each author
+	git blame --line-porcelain file |
+	sed -n 's/^author //p' |
+	sort | uniq -c | sort -rn
+
 
 SPECIFYING RANGES
 -----------------