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>