Fix error when inside a bare git repository

For example, "git diff" would print
"fatal: this operation must be run in a work tree"

We could still run git_branch_name in bare repositories in future.

lib/git/git_is_repo.fish

@@ -1,3 +1,7 @@
 function git_is_repo -d "Check if directory is a repository"
-  test -d .git; or command git rev-parse --git-dir >/dev/null 2> /dev/null
+  test -d .git
+  or begin
+    set -l info (command git rev-parse --git-dir --is-bare-repository 2>/dev/null)
+    and test $info[2] = false
+  end
 end