Fixed: subrepo push fails in submodule

The creation of the temporary worktree failed because the temporary
path was hard coded to be under the folder .git,
which does not exist for submodules (or worktrees, in fact).
Replaced it with a call to git rev-parse --git-dir, which supports
all scenarios.

Author: OskiKervinen-MF

lib/git-subrepo

@@ -28,7 +28,7 @@ bash+:import :std can
 
 VERSION=0.4.0
 REQUIRED_GIT_VERSION=2.7.0
-GIT_TMP=.git/tmp
+GIT_TMP="$(git rev-parse --git-dir)/tmp"
 
 # `git rev-parse` turns this into a getopt parser and a command usage message:
 GETOPT_SPEC="\