handle_alias: provide GIT_PREFIX to !alias

Provide an environment variable GIT_PREFIX which contains the subdirectory
from which a !alias was called (i.e. 'git rev-parse --show-prefix') since
these cd to the to level directory before they are executed.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -589,6 +589,8 @@ it will be treated as a shell command.  For example, defining
 "gitk --all --not ORIG_HEAD".  Note that shell commands will be
 executed from the top-level directory of a repository, which may
 not necessarily be the current directory.
+'GIT_PREFIX' is set as returned by running 'git rev-parse --show-prefix'
+from the original current directory. See linkgit:git-rev-parse[1].
 
 am.keepcr::
 	If true, git-am will call git-mailsplit for patches in mbox format

git.c

@@ -179,6 +179,8 @@ static int handle_alias(int *argcp, const char ***argv)
 		if (alias_string[0] == '!') {
 			const char **alias_argv;
 			int argc = *argcp, i;
+			struct strbuf sb = STRBUF_INIT;
+			const char *env[2];
 
 			commit_pager_choice();
 
@@ -189,7 +191,13 @@ static int handle_alias(int *argcp, const char ***argv)
 				alias_argv[i] = (*argv)[i];
 			alias_argv[argc] = NULL;
 
-			ret = run_command_v_opt(alias_argv, RUN_USING_SHELL);
+			strbuf_addstr(&sb, "GIT_PREFIX=");
+			if (subdir)
+				strbuf_addstr(&sb, subdir);
+			env[0] = sb.buf;
+			env[1] = NULL;
+			ret = run_command_v_opt_cd_env(alias_argv, RUN_USING_SHELL, NULL, env);
+			strbuf_release(&sb);
 			if (ret >= 0)   /* normal exit */
 				exit(ret);
 

t/t1020-subdirectory.sh

@@ -129,6 +129,16 @@ test_expect_success '!alias expansion' '
 	test_cmp expect actual
 '
 
+test_expect_success 'GIT_PREFIX for !alias' '
+	printf "dir/" >expect &&
+	(
+		git config alias.test "!sh -c \"printf \$GIT_PREFIX\"" &&
+		cd dir &&
+		git test >../actual
+	) &&
+	test_cmp expect actual
+'
+
 test_expect_success 'no file/rev ambiguity check inside .git' '
 	git commit -a -m 1 &&
 	(