branch -v: honor core.abbrev

Use the value from 'core.abbrev' configuration variable unless user
specifies the length on command line when showing commit object name
in "branch -v" output.

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

Documentation/git-branch.txt

@@ -113,7 +113,8 @@ OPTIONS
 
 --abbrev=<length>::
 	Alter the sha1's minimum display length in the output listing.
-	The default value is 7.
+	The default value is 7 and can be overridden by the `core.abbrev`
+	config option.
 
 --no-abbrev::
 	Display the full sha1s in the output listing rather than abbreviating them.

builtin/branch.c

@@ -613,7 +613,7 @@ static int opt_parse_merge_filter(const struct option *opt, const char *arg, int
 int cmd_branch(int argc, const char **argv, const char *prefix)
 {
 	int delete = 0, rename = 0, force_create = 0;
-	int verbose = 0, abbrev = DEFAULT_ABBREV, detached = 0;
+	int verbose = 0, abbrev = -1, detached = 0;
 	int reflog = 0;
 	enum branch_track track;
 	int kinds = REF_LOCAL_BRANCH;
@@ -696,6 +696,9 @@ int cmd_branch(int argc, const char **argv, const char *prefix)
 	if (!!delete + !!rename + !!force_create > 1)
 		usage_with_options(builtin_branch_usage, options);
 
+	if (abbrev == -1)
+		abbrev = DEFAULT_ABBREV;
+
 	if (delete)
 		return delete_branches(argc, argv, delete > 1, kinds);
 	else if (argc == 0)