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>

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