git-gui: Better handling of detached HEAD

If the current branch is not a symbolic-ref that points to a
name in the refs/heads/ namespace we now just assume that the
head is a detached head.  In this case we return the special
branch name of HEAD rather than empty string, as HEAD is a
valid revision specification and the empty string is not.

I have also slightly improved the current-branch function by
using string functions to parse the symbolic-ref data.  This
should be slightly faster than using a regsub.  I think the
code is clearer too.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -285,14 +285,24 @@ proc git {args} {
 }
 
 proc current-branch {} {
-	set ref {}
 	set fd [open [gitdir HEAD] r]
-	if {[gets $fd ref] <16
-	 || ![regsub {^ref: refs/heads/} $ref {} ref]} {
+	if {[gets $fd ref] < 1} {
 		set ref {}
 	}
 	close $fd
-	return $ref
+
+	set pfx {ref: refs/heads/}
+	set len [string length $pfx]
+	if {[string equal -length $len $pfx $ref]} {
+		# We're on a branch.  It might not exist.  But
+		# HEAD looks good enough to be a branch.
+		#
+		return [string range $ref $len end]
+	} else {
+		# Assume this is a detached head.
+		#
+		return HEAD
+	}
 }
 
 auto_load tk_optionMenu