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>