[PATCH v2 08/13] bash prompt: use bash builtins to find out current branch

["SZEDER Gábor" <szeder@ira.uka.de>]

From: SZEDER Gábor <szeder@ira.uka.de>

__git_ps1() runs the '$(git symbolic-ref HEAD)' command substitution
to find out whether we are on a branch and to find out the name of
that branch.  This imposes the overhead of fork()ing a subshell and
fork()+exec()ing a git process.

Since HEAD is in most cases a single-line file and the symbolic ref
format is quite simple to recognize and parse, read and parse it using
only bash builtins, thereby sparing all that fork()+exec() overhead.
However, HEAD can also be a symlink symbolic ref (due to
'core.preferSymlinkRefs'), so do this only if HEAD is not a symlink.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
---
 contrib/completion/git-prompt.sh | 46 ++++++++++++++++++++++++----------------
 1 file changed, 28 insertions(+), 18 deletions(-)

diff --git a/contrib/completion/git-prompt.sh b/contrib/completion/git-prompt.sh
index 26380787..4e5c8efa 100644
--- a/contrib/completion/git-prompt.sh
+++ b/contrib/completion/git-prompt.sh
@@ -355,25 +355,35 @@ __git_ps1 ()
 			r="|BISECTING"
 		fi
 
-		test -n "$b" ||
-		b="$(git symbolic-ref HEAD 2>/dev/null)" || {
-			detached=yes
-			b="$(
-			case "${GIT_PS1_DESCRIBE_STYLE-}" in
-			(contains)
-				git describe --contains HEAD ;;
-			(branch)
-				git describe --contains --all HEAD ;;
-			(describe)
-				git describe HEAD ;;
-			(* | default)
-				git describe --tags --exact-match HEAD ;;
-			esac 2>/dev/null)" ||
+		if [ -n "$b" ]; then
+			:
+		elif [ -h "$g/HEAD" ]; then
+			# symlink symbolic ref
+			b="$(git symbolic-ref HEAD 2>/dev/null)"
+		else
+			local head=""
+			read head 2>/dev/null <"$g/HEAD" || return
+			# is it a symbolic ref?
+			b="${head#ref: }"
+			if [ "$head" = "$b" ]; then
+				detached=yes
+				b="$(
+				case "${GIT_PS1_DESCRIBE_STYLE-}" in
+				(contains)
+					git describe --contains HEAD ;;
+				(branch)
+					git describe --contains --all HEAD ;;
+				(describe)
+					git describe HEAD ;;
+				(* | default)
+					git describe --tags --exact-match HEAD ;;
+				esac 2>/dev/null)" ||
 
-			b="$(cut -c1-7 "$g/HEAD" 2>/dev/null)..." ||
-			b="unknown"
-			b="($b)"
-		}
+				b="$(cut -c1-7 "$g/HEAD" 2>/dev/null)..." ||
+				b="unknown"
+				b="($b)"
+			fi
+		fi
 	fi
 
 	if [ -n "$step" ] && [ -n "$total" ]; then
-- 
1.8.3.1.487.g8f4672d