[PATCH] blame: default to HEAD in a bare repo when no start commit is given

["SZEDER Gábor" <szeder.dev@gmail.com>]

When 'git blame' is invoked without specifying the commit to start
blaming from, it starts from the given file's state in the work tree.
However, when invoked in a bare repository without a start commit,
then there is no work tree state to start from, and it dies with the
following error message:

  $ git rev-parse --is-bare-repository
  true
  $ git blame file.c
  fatal: this operation must be run in a work tree

This is misleading, because it implies that 'git blame' doesn't work
in bare repositories at all, but it does, in fact, work just fine when
it is given a commit to start from.

We could improve the error message, of course, but let's just default
to HEAD in a bare repository instead, as most likely that is what the
user wanted anyway (if they wanted to start from an other commit, then
they would have specified that in the first place).

'git annotate' is just a thin wrapper around 'git blame', so in the
same situation it printed the same misleading error message, and this
patch fixes it, too.

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
---
 builtin/blame.c     | 13 +++++++++++++
 t/annotate-tests.sh |  8 ++++++++
 2 files changed, 21 insertions(+)

diff --git a/builtin/blame.c b/builtin/blame.c
index 177c1022a0..21cde57e71 100644
--- a/builtin/blame.c
+++ b/builtin/blame.c
@@ -27,6 +27,7 @@
 #include "object-store.h"
 #include "blame.h"
 #include "string-list.h"
+#include "refs.h"
 
 static char blame_usage[] = N_("git blame [<options>] [<rev-opts>] [<rev>] [--] <file>");
 
@@ -993,6 +994,18 @@ int cmd_blame(int argc, const char **argv, const char *prefix)
 
 	revs.disable_stdin = 1;
 	setup_revisions(argc, argv, &revs, NULL);
+	if (!revs.pending.nr && is_bare_repository()) {
+		struct commit *head_commit;
+		struct object_id head_oid;
+
+		if (!resolve_ref_unsafe("HEAD", RESOLVE_REF_READING,
+					&head_oid, NULL) ||
+		    !(head_commit = lookup_commit_reference_gently(revs.repo,
+							     &head_oid, 1)))
+			die("no such ref: HEAD");
+
+		add_pending_object(&revs, &head_commit->object, "HEAD");
+	}
 
 	init_scoreboard(&sb);
 	sb.revs = &revs;
diff --git a/t/annotate-tests.sh b/t/annotate-tests.sh
index 6da48a2e0a..d933af5714 100644
--- a/t/annotate-tests.sh
+++ b/t/annotate-tests.sh
@@ -68,6 +68,14 @@ test_expect_success 'blame 1 author' '
 	check_count A 2
 '
 
+test_expect_success 'blame in a bare repo without starting commit' '
+	git clone --bare . bare.git &&
+	(
+		cd bare.git &&
+		check_count A 2
+	)
+'
+
 test_expect_success 'blame by tag objects' '
 	git tag -m "test tag" testTag &&
 	git tag -m "test tag #2" testTag2 testTag &&
-- 
2.21.0.539.g07239c3a71.dirty