fix(is-git-repo): recognize bare repositories

is_git_repo() used `git rev-parse --show-toplevel` to detect whether
the current directory is inside a git repository. --show-toplevel
fails for bare repositories since they have no working tree, so every
command that relies on this shared helper (e.g. `git browse`) reports
"Not a git repo!" when run from inside a bare repo, even though it
plainly is one.

Switch to `git rev-parse --git-dir`, which succeeds for both normal
and bare repositories and keeps the existing "not a repo" behavior
for non-repo directories.

Fixes #1238, reported and LGTM'd by maintainers there with this exact
fix; no PR had been opened for it yet.

Adds tests/is-git-repo.bats covering: a normal repo, a bare repo, and
a plain (non-repo) directory.

helper/is-git-repo

@@ -3,7 +3,9 @@
 #
 
 is_git_repo() {
-  git rev-parse --show-toplevel > /dev/null 2>&1
+  # --git-dir succeeds for both normal and bare repositories, whereas
+  # --show-toplevel fails inside a bare repository (it has no working tree).
+  git rev-parse --git-dir > /dev/null 2>&1
   result=$?
   if test $result != 0; then
     >&2 echo 'Not a git repo!'

tests/is-git-repo.bats

@@ -0,0 +1,32 @@
+# shellcheck shell=bash
+
+source "$BATS_TEST_DIRNAME/test_util.sh"
+
+setup_file() {
+	test_util.setup_file
+}
+
+setup() {
+	test_util.cd_test
+}
+
+@test "is_git_repo succeeds inside a normal (non-bare) repository" {
+	test_util.git_init
+
+	run bash "$BATS_TEST_DIRNAME/../helper/is-git-repo"
+	assert_success
+}
+
+@test "is_git_repo succeeds inside a bare repository (see #1238)" {
+	git init --bare --initial-branch main repo.git
+	cd repo.git
+
+	run bash "$BATS_TEST_DIRNAME/../helper/is-git-repo"
+	assert_success
+}
+
+@test "is_git_repo fails outside of a repository" {
+	run bash "$BATS_TEST_DIRNAME/../helper/is-git-repo"
+	assert_failure
+	assert_output 'Not a git repo!'
+}