show-branch -g: default to the current branch.

Now we have a separate reflog on HEAD, show-branch -g without an explicit
parameter defaults to the current branch, or HEAD when it is detached
from branches.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-show-branch.txt

@@ -11,7 +11,7 @@ SYNOPSIS
 'git-show-branch' [--all] [--remotes] [--topo-order] [--current]
 		[--more=<n> | --list | --independent | --merge-base]
 		[--no-name | --sha1-name] [--topics] [<rev> | <glob>]...
-'git-show-branch' (-g|--reflog)[=<n>[,<base>]] [--list] <ref>
+'git-show-branch' (-g|--reflog)[=<n>[,<base>]] [--list] [<ref>]
 
 DESCRIPTION
 -----------
@@ -97,11 +97,13 @@ OPTIONS
 	will show the revisions given by "git rev-list {caret}master
 	topic1 topic2"
 
---reflog[=<n>[,<base>]] <ref>::
+--reflog[=<n>[,<base>]] [<ref>]::
 	Shows <n> most recent ref-log entries for the given
 	ref.  If <base> is given, <n> entries going back from
 	that entry.  <base> can be specified as count or date.
-	`-g` can be used as a short-hand for this option.
+	`-g` can be used as a short-hand for this option.  When
+	no explicit <ref> parameter is given, it defaults to the
+	current branch (or `HEAD` if it is detached).
 
 Note that --more, --list, --independent and --merge-base options
 are mutually exclusive.

builtin-show-branch.c

@@ -690,7 +690,10 @@ int cmd_show_branch(int ac, const char **av, const char *prefix)
 
 		if (ac == 0) {
 			static const char *fake_av[2];
-			fake_av[0] = "HEAD";
+			const char *refname;
+
+			refname = resolve_ref("HEAD", sha1, 1, NULL);
+			fake_av[0] = xstrdup(refname);
 			fake_av[1] = NULL;
 			av = fake_av;
 			ac = 1;