Fix build failure on Windows

* src/regcomp.c: Drop usage of '__call__' in generation of error
  message.

* misc/regcomp-pass-R-CMD-check-git2r.patch: Updated

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Rename test cases

* Rename tests/collaboration.R to tests/push.R

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Add push test cases

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>