bug#32549: Allow passing custom options to vc-git-grep

[Federico Tedin <federicotedin@gmail.com>]

> Doesn't projectile-grep prompt you for a regexp?  I don't have it
> installed here, but it looks like you have to pass a non-nil ARG to get
> prompted for filenames, i.e.,
>
>   (let ((current-prefix-arg '(4)))
>     (projectile-grep nil t))

My mistake. Calling projectile-grep with current-prefix-arg '(4) indeed prompts
for the git-grep command _and_ a regexp.

> > Is there any way to set a default set of arguments in order to avoid
> > typing them when invoking the function?
>
> Not sure I follow, when you pass "something" and "*.el" to the function,
> those get inserted into the default command so you don't have to type
> them interactively.

I am sorry if I wasn't clear enough. What I'm trying to say is, when calling
vc-git-grep directly or indirectly with current-prefix-arg '(4), I am always
prompted for the full git-grep command. I would like to skip this step
completely,
and always use a custom set of arguments (like, for example, "-C 3"). Then, I
would be prompted for a regexp and (maybe) files, which would be added to this
custom git-grep command's arguments (this is how the rgrep command works, I
think).