commit: do not lose SQUASH_MSG contents

When concluding a conflicted "git merge --squash", the command
failed to read SQUASH_MSG that was prepared by "git merge", and
showed only the "# Conflicts:" list of conflicted paths.

Place the contents from SQUASH_MSG at the beginning, just like we
show the commit log skeleton first when concluding a normal merge,
and then show the "# Conflicts:" list, to help the user write the
log message for the resulting commit.

Test by Junio C Hamano <gitster@pobox.com>.

Signed-off-by: Sven Strickroth <sven@cs-ware.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/commit.c b/builtin/commit.c
index 4cbd5ff..6163972 100644
--- a/builtin/commit.c
+++ b/builtin/commit.c
@@ -726,9 +726,18 @@
 				      &sb, &ctx);
 		hook_arg1 = "message";
 	} else if (!stat(git_path_merge_msg(), &statbuf)) {
+		/*
+		 * prepend SQUASH_MSG here if it exists and a
+		 * "merge --squash" was originally performed
+		 */
+		if (!stat(git_path_squash_msg(), &statbuf)) {
+			if (strbuf_read_file(&sb, git_path_squash_msg(), 0) < 0)
+				die_errno(_("could not read SQUASH_MSG"));
+			hook_arg1 = "squash";
+		} else
+			hook_arg1 = "merge";
 		if (strbuf_read_file(&sb, git_path_merge_msg(), 0) < 0)
 			die_errno(_("could not read MERGE_MSG"));
-		hook_arg1 = "merge";
 	} else if (!stat(git_path_squash_msg(), &statbuf)) {
 		if (strbuf_read_file(&sb, git_path_squash_msg(), 0) < 0)
 			die_errno(_("could not read SQUASH_MSG"));