clang-format version detection is slightly broken (and definitely so for trunk builds)

[malcolmr]

At https://github.com/google/vim-codefmt/blob/293c208/autoload/codefmt/clangformat.vim#L32-34, we have this code:

    let l:version_string = matchstr(l:version_output, '\v\d+(.\d+)+')
    " If no version string was matched, cached version will be an empty list.
    let s:clang_format_version = map(split(l:version_string, '\.'), 'v:val + 0')

which is trying to find a version number (possibly "1.2.3" or "1.2" or just "1") in the output of clang-format --version:

$ clang-format --version
clang-format version 7.0.1-8+deb10u2 (tags/RELEASE_701/final)

However, for trunk builds, the output looks something like:

 clang-format --version
clang-format version mainline (4321b9f2e9842982d13234920a643e3a4657c60b)

(where "mainline" can be any string the configurer chooses).

However:

• matchstr(l:version_output, '\v\d+(.\d+)+') has . matching any character. That probably was intended to be a literal (\.) instead, otherwise the pattern could just have been \v\d.+.
• As a result, we consider any trunk build to be a version "number" containing the numeric start of the hex string (i.e. version 4321 in the above example).
• This means that we usually consider trunk builds to have all features (good), but only if their version string starts with enough non-zero/non-hex digits (bad).

We probably should do something like:

1. Change the regexp to use \. instead to match a literal.
2. Consider a non-standard version to always have the feature rather than not have it (at https://github.com/google/vim-codefmt/blob/293c208/autoload/codefmt/clangformat.vim#L38.

[dbarnett]

For "2. Consider a non-standard version to always have the feature" — That's just to say we have no idea, so we'll err on the side of trying to use the new feature?

[malcolmr]

Right, if we can't tell, then assume that the feature is present.