From: SZEDER Gábor <sze...@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 <sze...@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

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