[PATCH] refspec: allow @ on the left-hand side of refspecs

["brian m. carlson" <sandals@crustytoothpaste.net>]

The object ID parsing machinery is aware of "@" as a synonym for "HEAD"
and this is documented accordingly in gitrevisions(7).  The push
documentation describes the source portion of a refspec as "any
arbitrary 'SHA-1 expression'"; however, "@" is not allowed on the
left-hand side of a refspec, since we attempt to check for it being a
valid ref name and fail (since it is not).

Teach the refspec machinery about this alias and silently substitute
"HEAD" when we see "@".  This handles the fact that HEAD is a symref and
preserves its special behavior.  We need not handle other arbitrary
object ID expressions (such as "@^") when pushing because the revision
machinery already handles that for us.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
---
I probably type "git push upstream HEAD" from five to thirty times a
day, many of those where I typo "HEAD", so I decided to implement the
shorter form.  This design handles @ as HEAD in both fetch and push,
whereas alternate solutions would not.

check_refname_format explicitly rejects "@"; I tried at first to simply
ignore that with a flag, but we end up calling that from several other
places in the codebase and rejecting it and all of those places would
have needed updating.

I thought about putting the if/else logic in a function, but since it's
just four lines, I decided not to.  However, if people think it would be
tidier, I can do so.

Note that the test portion of the patch is best read with git diff -w;
the current version is very noisy.

 refspec.c             |   6 ++-
 t/t5516-fetch-push.sh | 104 +++++++++++++++++++++---------------------
 2 files changed, 58 insertions(+), 52 deletions(-)

diff --git a/refspec.c b/refspec.c
index e8010dce0c..57c2f65104 100644
--- a/refspec.c
+++ b/refspec.c
@@ -62,8 +62,12 @@ static int parse_refspec(struct refspec_item *item, const char *refspec, int fet
 		return 0;
 	}
 
+	if (llen == 1 && lhs[0] == '@')
+		item->src = xstrdup("HEAD");
+	else
+		item->src = xstrndup(lhs, llen);
+
 	item->pattern = is_glob;
-	item->src = xstrndup(lhs, llen);
 	flags = REFNAME_ALLOW_ONELEVEL | (is_glob ? REFNAME_REFSPEC_PATTERN : 0);
 
 	if (fetch) {
diff --git a/t/t5516-fetch-push.sh b/t/t5516-fetch-push.sh
index a5077d8b7c..cbccbd2f8d 100755
--- a/t/t5516-fetch-push.sh
+++ b/t/t5516-fetch-push.sh
@@ -436,70 +436,72 @@ test_expect_success 'push ref expression with non-existent, incomplete dest' '
 
 '
 
-test_expect_success 'push with HEAD' '
+for ref in HEAD @
+do
+	test_expect_success "push with $ref" '
 
-	mk_test testrepo heads/master &&
-	git checkout master &&
-	git push testrepo HEAD &&
-	check_push_result testrepo $the_commit heads/master
+		mk_test testrepo heads/master &&
+		git checkout master &&
+		git push testrepo $ref &&
+		check_push_result testrepo $the_commit heads/master
 
-'
+	'
 
-test_expect_success 'push with HEAD nonexisting at remote' '
+	test_expect_success "push with $ref nonexisting at remote" '
 
-	mk_test testrepo heads/master &&
-	git checkout -b local master &&
-	git push testrepo HEAD &&
-	check_push_result testrepo $the_commit heads/local
-'
+		mk_test testrepo heads/master &&
+		git checkout -B local master &&
+		git push testrepo $ref &&
+		check_push_result testrepo $the_commit heads/local
+	'
 
-test_expect_success 'push with +HEAD' '
+	test_expect_success "push with +$ref" '
 
-	mk_test testrepo heads/master &&
-	git checkout master &&
-	git branch -D local &&
-	git checkout -b local &&
-	git push testrepo master local &&
-	check_push_result testrepo $the_commit heads/master &&
-	check_push_result testrepo $the_commit heads/local &&
+		mk_test testrepo heads/master &&
+		git checkout master &&
+		git branch -D local &&
+		git checkout -b local &&
+		git push testrepo master local &&
+		check_push_result testrepo $the_commit heads/master &&
+		check_push_result testrepo $the_commit heads/local &&
 
-	# Without force rewinding should fail
-	git reset --hard HEAD^ &&
-	test_must_fail git push testrepo HEAD &&
-	check_push_result testrepo $the_commit heads/local &&
+		# Without force rewinding should fail
+		git reset --hard HEAD^ &&
+		test_must_fail git push testrepo $ref &&
+		check_push_result testrepo $the_commit heads/local &&
 
-	# With force rewinding should succeed
-	git push testrepo +HEAD &&
-	check_push_result testrepo $the_first_commit heads/local
+		# With force rewinding should succeed
+		git push testrepo +$ref &&
+		check_push_result testrepo $the_first_commit heads/local
 
-'
+	'
 
-test_expect_success 'push HEAD with non-existent, incomplete dest' '
+	test_expect_success "push $ref with non-existent, incomplete dest" '
 
-	mk_test testrepo &&
-	git checkout master &&
-	git push testrepo HEAD:branch &&
-	check_push_result testrepo $the_commit heads/branch
+		mk_test testrepo &&
+		git checkout master &&
+		git push testrepo $ref:branch &&
+		check_push_result testrepo $the_commit heads/branch
 
-'
+	'
 
-test_expect_success 'push with config remote.*.push = HEAD' '
-
-	mk_test testrepo heads/local &&
-	git checkout master &&
-	git branch -f local $the_commit &&
-	(
-		cd testrepo &&
-		git checkout local &&
-		git reset --hard $the_first_commit
-	) &&
-	test_config remote.there.url testrepo &&
-	test_config remote.there.push HEAD &&
-	test_config branch.master.remote there &&
-	git push &&
-	check_push_result testrepo $the_commit heads/master &&
-	check_push_result testrepo $the_first_commit heads/local
-'
+	test_expect_success "push with config remote.*.push = $ref" '
+		mk_test testrepo heads/local &&
+		git checkout master &&
+		git branch -f local $the_commit &&
+		(
+			cd testrepo &&
+			git checkout local &&
+			git reset --hard $the_first_commit
+		) &&
+		test_config remote.there.url testrepo &&
+		test_config remote.there.push $ref &&
+		test_config branch.master.remote there &&
+		git push &&
+		check_push_result testrepo $the_commit heads/master &&
+		check_push_result testrepo $the_first_commit heads/local
+	'
+done
 
 test_expect_success 'push with remote.pushdefault' '
 	mk_test up_repo heads/master &&