commit: use value of GIT_REFLOG_ACTION env variable as reflog message

The environment variable GIT_REFLOG_ACTION was used by git-commit.sh,
but when it was converted to a builtin
(f5bbc3225c, Port git commit to C,
Nov 8 2007) this was lost.

Let's use it again as it is more user friendly when reverting or
cherry-picking to see "revert" or "cherry-pick" in the reflog rather
than to just see "commit".

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Acked-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/commit.c

@@ -1232,12 +1232,15 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 	}
 
 	/* Determine parents */
+	reflog_msg = getenv("GIT_REFLOG_ACTION");
 	if (initial_commit) {
+		if (!reflog_msg)
 			reflog_msg = "commit (initial)";
 	} else if (amend) {
 		struct commit_list *c;
 		struct commit *commit;
 
+		if (!reflog_msg)
 			reflog_msg = "commit (amend)";
 		commit = lookup_commit(head_sha1);
 		if (!commit || parse_commit(commit))
@@ -1249,6 +1252,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		struct strbuf m = STRBUF_INIT;
 		FILE *fp;
 
+		if (!reflog_msg)
 			reflog_msg = "commit (merge)";
 		pptr = &commit_list_insert(lookup_commit(head_sha1), pptr)->next;
 		fp = fopen(git_path("MERGE_HEAD"), "r");
@@ -1272,6 +1276,7 @@ int cmd_commit(int argc, const char **argv, const char *prefix)
 		if (allow_fast_forward)
 			parents = reduce_heads(parents);
 	} else {
+		if (!reflog_msg)
 			reflog_msg = "commit";
 		pptr = &commit_list_insert(lookup_commit(head_sha1), pptr)->next;
 	}

t/t3501-revert-cherry-pick.sh

@@ -47,7 +47,8 @@ test_expect_success 'cherry-pick after renaming branch' '
 	git cherry-pick added &&
 	test $(git rev-parse HEAD^) = $(git rev-parse rename2) &&
 	test -f opos &&
-	grep "Add extra line at the end" opos
+	grep "Add extra line at the end" opos &&
+	git reflog -1 | grep cherry-pick
 
 '
 
@@ -57,7 +58,8 @@ test_expect_success 'revert after renaming branch' '
 	git revert added &&
 	test $(git rev-parse HEAD^) = $(git rev-parse rename1) &&
 	test -f spoo &&
-	! grep "Add extra line at the end" spoo
+	! grep "Add extra line at the end" spoo &&
+	git reflog -1 | grep revert
 
 '