added option to only show local commits

[Teszko]

Added an option -l/--local to show only commits that are ahead of origin/{branch}.
For this I introduced the function set_log_command() to select a different command template as it makes little sense to filter author and date in combination with -local, at least by default. The command is expected to show ALL commits ahead of the remote branch.

One thing I don't like is the error handling for the git log command.
COMMITS=($(eval ${GIT_LOG} 2>/dev/null))
when using the -local option and the branch origin/{branch} does not exist yet (as is the case if you
just created the new branch and didn't push anything yet) the error does not get caught and instead
the "did nothing during this period" message is shown. So error handling should be added, but
that goes beyond this PR.

[Teszko]

• updated documentation.