blame: add --abbrev command line option and make it honor core.abbrev

If user sets config.abbrev option, use it as if --abbrev was given.  This
is the default value and user can override different abbrev length by
specifying the --abbrev=N command line option.

Signed-off-by: Namhyung Kim <namhyung@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-blame.txt

@@ -9,7 +9,7 @@ SYNOPSIS
 --------
 [verse]
 'git blame' [-c] [-b] [-l] [--root] [-t] [-f] [-n] [-s] [-e] [-p] [-w] [--incremental] [-L n,m]
-	    [-S <revs-file>] [-M] [-C] [-C] [-C] [--since=<date>]
+	    [-S <revs-file>] [-M] [-C] [-C] [-C] [--since=<date>] [--abbrev=<n>]
 	    [<rev> | --contents <file> | --reverse <rev>] [--] <file>
 
 DESCRIPTION
@@ -73,6 +73,11 @@ include::blame-options.txt[]
 	Ignore whitespace when comparing the parent's version and
 	the child's to find where the lines came from.
 
+--abbrev=<n>::
+	Instead of using the default 7+1 hexadecimal digits as the
+	abbreviated object name, use <n>+1 digits. Note that 1 column
+	is used for a caret to mark the boundary commit.
+
 
 THE PORCELAIN FORMAT
 --------------------

builtin/blame.c

@@ -41,6 +41,7 @@ static int reverse;
 static int blank_boundary;
 static int incremental;
 static int xdl_opts;
+static int abbrev = -1;
 
 static enum date_mode blame_date_mode = DATE_ISO8601;
 static size_t blame_date_width;
@@ -1670,7 +1671,7 @@ static void emit_other(struct scoreboard *sb, struct blame_entry *ent, int opt)
 	cp = nth_line(sb, ent->lno);
 	for (cnt = 0; cnt < ent->num_lines; cnt++) {
 		char ch;
-		int length = (opt & OUTPUT_LONG_OBJECT_NAME) ? 40 : 8;
+		int length = (opt & OUTPUT_LONG_OBJECT_NAME) ? 40 : abbrev;
 
 		if (suspect->commit->object.flags & UNINTERESTING) {
 			if (blank_boundary)
@@ -2310,6 +2311,7 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 		{ OPTION_CALLBACK, 'C', NULL, &opt, "score", "Find line copies within and across files", PARSE_OPT_OPTARG, blame_copy_callback },
 		{ OPTION_CALLBACK, 'M', NULL, &opt, "score", "Find line movements within and across files", PARSE_OPT_OPTARG, blame_move_callback },
 		OPT_CALLBACK('L', NULL, &bottomtop, "n,m", "Process only line range n,m, counting from 1", blame_bottomtop_callback),
+		OPT__ABBREV(&abbrev),
 		OPT_END()
 	};
 
@@ -2345,6 +2347,11 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 parse_done:
 	argc = parse_options_end(&ctx);
 
+	if (abbrev == -1)
+		abbrev = default_abbrev;
+	/* one more abbrev length is needed for the boundary commit */
+	abbrev++;
+
 	if (revs_file && read_ancestry(revs_file))
 		die_errno("reading graft file '%s' failed", revs_file);