checksrc-all.pl: skip non-repository files

[vszakats]

To avoid noise due to local C files when using automatic local checksrc
checks (e.g. via CMake -DCURL_LINT=ON option, or curl-lint target).

Also replace single-quote with double-quote in external git command, for
portability.

Follow-up to 88ff396 #17882
Follow-up to e785e89 #17376

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https://github.com/curl/curl/pull/20439/files?w=1

[dfandrich]

Doesn't this mean that new files won't get checked until they're committed? A developer could run all the tests, see that they're clean, then commit and push them upstream only to have CI complain. Could this instead check only C files that are a part of the build rather than those that are in git? How often are there actually C files in the curl source tree that devs want to skip?

[vszakats]

Doesn't this mean that new files won't get checked until they're committed? A developer could run all the tests, see that they're clean, then commit and push them upstream only to have CI complain. Could this instead check only C files that are a part of the build rather than those that are in git? How often are there actually C files in the curl source tree that devs want to skip?

It means new files need to be git add-ed to be checked, yes. (but not
necessarily committed)

Speaking for myself, some misc C files are in the source tree all the time: Test
snippets, backup/previous/next/master versions for diffs, 3rd-party headers,
etc. It made me avoid automatic checksrc, and only used it occasionally on
individual files, which means issues were only caught in CI most of the time.

In my experience adding new C files happens less often than updating existing
ones. I guess it depends on personal workflow; for new files I tend to git add
and commit early, to track subsequent edits. (Also not to forget adding new files
at all, which happened quite a few times in early Git days!)

I expect this to allow running checksrc routinely or automatically, to catch
checksrc issues locally, without CI, in most cases.

That said, checksrc works differently with autotools: it's not using this script.
It also doesn't use a filesystem glob as this script did before this patch. Instead
it uses file lists from Makefile.inc/Makefile.am. This is more accurate than
git ls-files and also works outside a git sandbox. It also seems complex to
implement in Perl.

edit: FWIW, outside a git sandbox, this script continues using the filesystem
glob, as before this patch.