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>

contrib/completion/git-prompt.sh

@@ -392,7 +392,7 @@ __git_ps1 ()
 					git describe --tags --exact-match HEAD ;;
 				esac 2>/dev/null)" ||
 
-				b="$(cut -c1-7 "$g/HEAD" 2>/dev/null)..." ||
+				b="$(git rev-parse --short HEAD 2>/dev/null)..." ||
 				b="unknown"
 				b="($b)"
 			}

t/t9903-bash-prompt.sh

@@ -50,7 +50,8 @@ test_expect_success SYMLINKS 'prompt - branch name - symlink symref' '
 '
 
 test_expect_success 'prompt - detached head' '
-	printf " ((%s...))" $(git log -1 --format="%h" b1^) >expected &&
+	printf " ((%s...))" $(git log -1 --format="%h" --abbrev=13 b1^) >expected &&
+	test_config core.abbrev 13 &&
 	git checkout b1^ &&
 	test_when_finished "git checkout master" &&
 	__git_ps1 >"$actual" &&