i18n: fix mismatched camelCase config variables

i18n: fix mismatched camelCase config variables

Some config variables are combinations of multiple words, and we
typically write them in camelCase forms in manpage and translatable
strings. It's not easy to find mismatches for these camelCase config
variables during code reviews, but occasionally they are identified
during localization translations.

To check for mismatched config variables, I introduced a new feature
in the helper program for localization[^1]. The following mismatched
config variables have been identified by running the helper program,
such as "git-po-helper check-pot".

Lowercase in manpage should use camelCase:

 * Documentation/config/http.txt: http.pinnedpubkey

Lowercase in translable strings should use camelCase:

 * builtin/fast-import.c:  pack.indexversion
 * builtin/gc.c:           gc.logexpiry
 * builtin/index-pack.c:   pack.indexversion
 * builtin/pack-objects.c: pack.indexversion
 * builtin/repack.c:       pack.writebitmaps
 * commit.c:               i18n.commitencoding
 * gpg-interface.c:        user.signingkey
 * http.c:                 http.postbuffer
 * submodule-config.c:     submodule.fetchjobs

Mismatched camelCases, choose the former:

 * Documentation/config/transfer.txt: transfer.credentialsInUrl
   remote.c:                          transfer.credentialsInURL

[^1]: https://github.com/git-l10n/git-po-helper

Signed-off-by: Jiang Xin <zhiyou.jx@alibaba-inc.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>