grep: stop modifying buffer in strip_timestamp

When grepping for headers in commit objects, we receive individual lines (e.g., "author Name <email> 1234 -0000"), and then strip off the timestamp to do our match. We do so by writing a NUL byte over the whitespace separator, and then remembering to restore it later. We had to do it this way when this was added back in a4d7d2c6db (log --author/--committer: really match only with name part, 2008-09-04), because we fed the result directly to regexec(), which expects a NUL-terminated string. But since b7d36ffca0 (regex: use regexec_buf(), 2016-09-21), we have a function which can match part of a buffer. So instead of modifying the string, we can instead just move the "eol" pointer, and the rest of the code will do the right thing. This will let further patches mark more buffers as "const". Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>