bash prompt: combine 'git rev-parse' for detached head

When describing a detached HEAD according to the $GIT_PS1_DESCRIBE
environment variable fails, __git_ps1() now runs the '$(git rev-parse
--short HEAD)' command substitution to get the abbreviated detached
HEAD commit object name.  This imposes the overhead of fork()ing a
subshell and fork()+exec()ing a git process.

Avoid this overhead by combining this command substitution with the
"main" 'git rev-parse' execution for getting the path to the .git
directory & co.  This means that we'll look for the abbreviated commit
object name even when it's not necessary, because we're on a branch or
the detached HEAD can be described.  It doesn't matter, however,
because once 'git rev-parse' is up and running to fulfill all those
other queries, the additional overhead of looking for the abbreviated
commit object name is not measurable because it's lost in the noise.

There is a caveat, however, when we are on an unborn branch, because
in that case HEAD doesn't point to a valid commit, hence the query for
the abbreviated commit object name fails.  Therefore, '--short HEAD'
must be the last options to 'git rev-parse' in order to get all the
other necessary information for the prompt even on an unborn branch.
Furthermore, in that case, and in that case only, 'git rev-parse'
doesn't output the last line containing the abbreviated commit object
name, obviously, so we have to take care to only parse it if 'git
rev-parse' exited without any error.

Although there are tests already excercising __git_ps1() on unborn
branches, they all do so implicitly.  Add a test that checks this
explicitly.

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

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