Trailers Containing Underscore or Dot Characters

[ふじを <ffjlabo@gmail.com>]

Hi everyone,
First of all, thank you for making such a tool. I am grateful to use
this tool every day.

I encountered the issue with trailers containing Underscore or Dot characters.
I don't know if this behavior is OK as a specification, or not. So I
created the post.

What did you do before the bug happened? (Steps to reproduce your issue)

I added coommit with trailers which have "_" or "." in each key.
```
git commit -m "Test" --trailer "test_hoge: fuga" --trailer "test.hoge:
fuga" --trailer "test-hoge: fuga"
```
What did you expect to happen? (Expected behavior)
I expcted that they are shown like this.
```
# git show
    Test
    test_hoge: fuga
    test.hoge: fuga
    test-hoge: fuga
```

What happened instead? (Actual behavior)

Actually, some of them are unexpected value.
```
# git show
commit 70afae811def2345bead6660e0f3183662c9f4df (HEAD -> master)
Author: Your Name <you@example.com>
Date:   Wed Nov 13 08:28:19 2024 +0300
    Test
    test_hoge: fuga:
    test.hoge: fuga:
    test-hoge: fuga
```

What's different between what you expected and what actually happened?

The one for `--trailer "test-hoge: fuga"` is finished with nothing.
But The others for ones which have "_" or "." are finished with ":".

Anything else you want to add:

I want to use such characters to add some key-value received other
service (e.g. GitHub Actions) as is.
Is there any chance that characters like _ and . will be allowed in
the trailer's key?

By the way, I tried the investigation for the current specification.
I hope this is helpful in some way.

[1] This behavior occured when executing `git interpret-trailers`.
```
# echo msg > msg
# git interpret-trailers --trailer "test_hoge: test" --trailer
"test.hoge: test" --trailer "test-hoge: test" --in-place msg
```
```
# cat msg
msg
test_hoge: test:
test.hoge: test:
test-hoge: test
```
[2] There might be the cause in the process of the persing trailers.
if they have such characters, the whole of the trailer option values
are interpreted as one token even though they have ":" if they have
such characters.

I checked the source code at
- parse_trailers_from_command_line_args
https://github.com/git/git/blob/564d0252ca632e0264ed670534a51d18a689ef5d/trailer.c#L748C1-L764C4
- -> find_separator
https://github.com/git/git/blob/564d0252ca632e0264ed670534a51d18a689ef5d/trailer.c#L614C1-L642C1

[System Info]
git version:
git version 2.43.0
cpu: aarch64
no commit associated with this build
sizeof-long: 8
sizeof-size_t: 8
shell-path: /bin/sh
uname: Linux 6.10.11-orbstack-00280-g1304bd068592 #21 SMP Sat Sep 21
10:45:28 UTC 2024 aarch64
compiler info: gnuc: 13.2
libc info: glibc: 2.39
$SHELL (typically, interactive shell): <unset>
[Enabled Hooks]