git-gui: Guess our version accurately as a subproject.

When we are included as a subproject, such as how git.git carries
us, we want to retain our own version number and not the version
number assigned by git.git's own tags.  Consequently we need to
locate the correct tag which applies to our tree content and
its commit lineage.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

GIT-VERSION-GEN

@@ -6,12 +6,45 @@ DEF_VER=0.5.GIT
 LF='
 '
 
-# First try git-describe, then see if there is a version file
-# (included in release tarballs), then default
-if VN=$(git describe --abbrev=4 HEAD 2>/dev/null) &&
+tree_search ()
+{
+	head=$1
+	tree=$2
+	for p in $(git rev-list --parents --max-count=1 $head 2>/devnull)
+	do
+		test $tree = $(git rev-parse $p^{tree} 2>/dev/null) &&
+		vn=$(git describe --abbrev=4 $p 2>/dev/null) &&
+		case "$vn" in
+		gitgui-[0-9]*) echo $vn; break;;
+		esac
+	done
+}
+
+# We may be a subproject, so try looking for the merge
+# commit that supplied this directory content if we are
+# not at the toplevel.  We probably will always be the
+# second parent in the commit, but we shouldn't rely on
+# that fact.
+#
+# If we are at the toplevel or the merge assumption fails
+# try looking for a gitgui-* tag, or fallback onto the
+# distributed version file.
+
+if prefix="$(git rev-parse --show-prefix 2>/dev/null)"
+   test -n "$prefix" &&
+   head=$(git rev-list --max-count=1 HEAD -- . 2>/dev/null) &&
+   tree=$(git rev-parse --verify "HEAD:$prefix" 2>/dev/null) &&
+   VN=$(tree_search $head $tree)
    case "$VN" in
-   *$LF*) (exit 1) ;;
    gitgui-[0-9]*) : happy ;;
+   *) (exit 1) ;;
+   esac
+then
+	VN=$(echo "$VN" | sed -e 's/^gitgui-//;s/-/./g');
+elif VN=$(git describe --abbrev=4 HEAD 2>/dev/null) &&
+   case "$VN" in
+   gitgui-[0-9]*) : happy ;;
+   *) (exit 1) ;;
    esac
 then
 	VN=$(echo "$VN" | sed -e 's/^gitgui-//;s/-/./g');