Refactor parse_loc

We want to use the same style of -L n,m argument for 'git log -L' as
for git-blame.  Refactor the argument parsing of the range arguments
from builtin/blame.c to the (new) file that will hold the 'git log -L'
logic.

To accommodate different data structures in blame and log -L, the file
contents are abstracted away; parse_range_arg takes a callback that it
uses to get the contents of a line of the (notional) file.

The new test is for a case that made me pause during debugging: the
'blame -L with invalid end' test was the only one that noticed an
outright failure to parse the end *at all*.  So make a more explicit
test for that.

Signed-off-by: Bo Yang <struggleyb.nku@gmail.com>
Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

line-range.h

@@ -0,0 +1,24 @@
+#ifndef LINE_RANGE_H
+#define LINE_RANGE_H
+
+/*
+ * Parse one item in an -L begin,end option w.r.t. the notional file
+ * object 'cb_data' consisting of 'lines' lines.
+ *
+ * The 'nth_line_cb' callback is used to determine the start of the
+ * line 'lno' inside the 'cb_data'.  The caller is expected to already
+ * have a suitable map at hand to make this a constant-time lookup.
+ *
+ * Returns 0 in case of success and -1 if there was an error.  The
+ * actual range is stored in *begin and *end.  The counting starts
+ * at 1!  In case of error, the caller should show usage message.
+ */
+
+typedef const char *(*nth_line_fn_t)(void *data, long lno);
+
+extern int parse_range_arg(const char *arg,
+			   nth_line_fn_t nth_line_cb,
+			   void *cb_data, long lines,
+			   long *begin, long *end);
+
+#endif /* LINE_RANGE_H */