bug#36644: Git log search

[Juri Linkov]

>> So I think we should treat this as we do in "M-x grep": leave the
>> semantics of PATTERN backend-dependent, and rely on the user to quote
>> some characters in it as needed.  Admittedly, 'grep' is lower-level
>> than 'vc-log-search', but at least we have a precedent.
>
> The difference is, 'M-x grep' doesn't use different backends. Although I'd
> be happy to see that capability.

grep backends can be switched implicitly by installing OS packages
like 'agrep', 'pcregrep', 'ripgrep', etc.