gitconfig: Trust Beyond Compare's exit code

See http://www.scootersoftware.com/support.php?zz=kb_vcs#gitwindows.
gitconfig: Add the "has" alias to list branches that contain a commit
git-set-author-committer: Do not require bash anymore
git-show-merge-resolution: Use cut instead of grep for MSYS compatibility

The current version 2.4.2 of MSYS grep does neither have "-P" nor "-o".
git-show-merge-resolution: Really use the configured merge tool

Rework the script to really use the configured merge tool instead of
diffuse and to properly show conflicting files only, or all files that
were concurrently changed in both parents.
Rework all exit codes to be non-negative for dash compatibility