Add current branch in PS1 support to git-completion.bash.

Many users want to display the current branch name of the current git
repository as part of their PS1 prompt, much as their PS1 prompt might
also display the current working directory name.

We don't force our own PS1 onto the user.  Instead we let them craft
their own PS1 string and offer them the function __git_ps1 which they
can invoke to obtain either "" (when not in a git repository) or
"(%s)" where %s is the name of the current branch, as read from HEAD,
with the leading refs/heads/ removed.
Signed-off-by: NShawn O. Pearce <spearce@spearce.org>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

contrib/completion/git-completion.bash

@@ -18,12 +18,31 @@
 #    2) Added the following line to your .bashrc:
 #        source ~/.git-completion.sh
 #
+#    3) Consider changing your PS1 to also show the current branch:
+#        PS1='[\u@\h \W$(__git_ps1 " (%s)")]\$ '
+#
+#       The argument to __git_ps1 will be displayed only if you
+#       are currently in a git repository.  The %s token will be
+#       the name of the current branch.
+#
 
 __gitdir ()
 {
 	echo "${__git_dir:-$(git rev-parse --git-dir 2>/dev/null)}"
 }
 
+__git_ps1 ()
+{
+	local b="$(git symbolic-ref HEAD 2>/dev/null)"
+	if [ -n "$b" ]; then
+		if [ -n "$1" ]; then
+			printf "$1" "${b##refs/heads/}"
+		else
+			printf " (%s)" "${b##refs/heads/}"
+		fi
+	fi
+}
+
 __git_refs ()
 {
 	local cmd i is_hash=y dir="${1:-$(__gitdir)}"