*config.txt: stick to camelCase naming convention

*config.txt: stick to camelCase naming convention

This should improve readability. Compare "thislongname" and
"thisLongName". The following keys are left in unchanged. We can
decide what to do with them later.

 - am.keepcr
 - core.autocrlf .safecrlf .trustctime
 - diff.dirstat .noprefix
 - gitcvs.usecrlfattr
 - gui.blamehistoryctx .trustmtime
 - pull.twohead
 - receive.autogc
 - sendemail.signedoffbycc .smtpsslcertpath .suppresscc

Helped-by: Eric Sunshine <redacted>
Signed-off-by: Nguyễn Thái Ngọc Duy <redacted>
Signed-off-by: Eric Sunshine <redacted>
Signed-off-by: Junio C Hamano <redacted>