bash prompt: use bash builtins to check for unborn branch for dirty state

When the dirty work tree and index status indicator is enabled,
__git_ps1() checks for changes in the index by running 'git diff-index
--cached --quiet HEAD --' and looking at its exit code.  However, that
makes sense only when HEAD points to a valid commit: on an unborn
branch the failure of said command would be caused by the invalid
HEAD, not by changes in the index.  Therefore, __git_ps1() first
checks for a valid HEAD by running 'git rev-parse --quiet --verify
HEAD'.

Since the previous patch we implicitly check HEAD's validity by
running 'git rev-parse ... --short HEAD', making the dirty status
indicator's 'git rev-parse' check redundant.  It's sufficient to check
for non-emptyness of the variable holding the abbreviated commit
object name, thereby sparing the overhead of fork()+exec()ing a git
process.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>

contrib/completion/git-prompt.sh

@@ -429,7 +429,7 @@ __git_ps1 ()
 		   [ "$(git config --bool bash.showDirtyState)" != "false" ]
 		then
 			git diff --no-ext-diff --quiet --exit-code || w="*"
-			if git rev-parse --quiet --verify HEAD >/dev/null; then
+			if [ -n "$short_sha" ]; then
 				git diff-index --cached --quiet HEAD -- || i="+"
 			else
 				i="#"