bash prompt: print unique detached HEAD abbreviated object name

bash prompt: print unique detached HEAD abbreviated object name

When describing a detached HEAD according to the $GIT_PS1_DESCRIBE
environment variable fails, __git_ps1() runs 'cut -c1-7 .git/HEAD' to
show the 7 hexdigits abbreviated commit object name in the prompt.
Obviously, this neither respects core.abbrev nor produces a unique
object name.

Fix this by using 'git rev-parse --short HEAD' instead and adjust the
corresponding test to use non-standard number of hexdigits.

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