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>

contrib/completion/git-prompt.sh

@@ -311,8 +311,12 @@ __git_ps1 ()
 		;;
 	esac
 
-	local repo_info="$(git rev-parse --git-dir --is-inside-git-dir \
+	local repo_info rev_parse_exit_code
-		--is-bare-repository --is-inside-work-tree 2>/dev/null)"
+	repo_info="$(git rev-parse --git-dir --is-inside-git-dir \
+		--is-bare-repository --is-inside-work-tree \
+		--short HEAD 2>/dev/null)"
+	rev_parse_exit_code="$?"
+
 	if [ -z "$repo_info" ]; then
 		if [ $pcmode = yes ]; then
 			#In PC mode PS1 always needs to be set
@@ -321,6 +325,11 @@ __git_ps1 ()
 		return
 	fi
 
+	local short_sha
+	if [ "$rev_parse_exit_code" = "0" ]; then
+		short_sha="${repo_info##*$'\n'}"
+		repo_info="${repo_info%$'\n'*}"
+	fi
 	local inside_worktree="${repo_info##*$'\n'}"
 	repo_info="${repo_info%$'\n'*}"
 	local bare_repo="${repo_info##*$'\n'}"
@@ -392,8 +401,7 @@ __git_ps1 ()
 					git describe --tags --exact-match HEAD ;;
 				esac 2>/dev/null)" ||
 
-				b="$(git rev-parse --short HEAD 2>/dev/null)..." ||
+				b="$short_sha..."
-				b="unknown"
 				b="($b)"
 			fi
 		fi

t/t9903-bash-prompt.sh

@@ -49,6 +49,14 @@ test_expect_success SYMLINKS 'prompt - branch name - symlink symref' '
 	test_cmp expected "$actual"
 '
 
+test_expect_success 'prompt - unborn branch' '
+	printf " (unborn)" >expected &&
+	git checkout --orphan unborn &&
+	test_when_finished "git checkout master" &&
+	__git_ps1 >"$actual" &&
+	test_cmp expected "$actual"
+'
+
 test_expect_success 'prompt - detached head' '
 	printf " ((%s...))" $(git log -1 --format="%h" --abbrev=13 b1^) >expected &&
 	test_config core.abbrev 13 &&