Improve bash prompt to detect various states like an unfinished merge

This patch makes the git prompt (when enabled) show if a merge or a
rebase is unfinished. It also detects if a bisect is being done as
well as detached checkouts.

An uncompleted git-am cannot be distinguised from a rebase (the
non-interactive version). Instead of having an even longer prompt
we simply ignore that and hope the power users that use git-am knows
the difference.

Signed-off-by: Robin Rosenberg <robin.rosenberg@dewire.com>
Acked-by: Shawn O. Pearce <spearce@spearce.org>

contrib/completion/git-completion.bash

@@ -64,12 +64,41 @@ __gitdir ()
 
 __git_ps1 ()
 {
-	local b="$(git symbolic-ref HEAD 2>/dev/null)"
-	if [ -n "$b" ]; then
-		if [ -n "$1" ]; then
-			printf "$1" "${b##refs/heads/}"
+	local g="$(git rev-parse --git-dir 2>/dev/null)"
+	if [ -n "$g" ]; then
+		local r
+		local b
+		if [ -d "$g/../.dotest" ]
+		then
+			r="|AM/REBASE"
+			b="$(git symbolic-ref HEAD 2>/dev/null)"
+		elif [ -f "$g/.dotest-merge/interactive" ]
+		then
+			r="|REBASE-i"
+			b="$(cat $g/.dotest-merge/head-name)"
+		elif [ -d "$g/.dotest-merge" ]
+		then
+			r="|REBASE-m"
+			b="$(cat $g/.dotest-merge/head-name)"
+		elif [ -f "$g/MERGE_HEAD" ]
+		then
+			r="|MERGING"
+			b="$(git symbolic-ref HEAD 2>/dev/null)"
 		else
-			printf " (%s)" "${b##refs/heads/}"
+			if [ -f $g/BISECT_LOG ]
+			then
+				r="|BISECTING"
+			fi
+			if ! b="$(git symbolic-ref HEAD 2>/dev/null)"
+			then
+				b="$(cut -c1-7 $g/HEAD)..."
+			fi
+		fi
+
+		if [ -n "$1" ]; then
+			printf "$1" "${b##refs/heads/}$r"
+		else
+			printf " (%s)" "${b##refs/heads/}$r"
 		fi
 	fi
 }