Regression: git no longer works with musl libc's regex impl

This commit broke support for using git with musl libc:

https://github.com/git/git/commit/2f8952250a84313b74f96abb7b035874854cf202

Rather than depending on non-portable GNU regex extensions, there is a
simple portable fix for the issue this code was added to work around:
When a text file is being mmapped for use with string functions which
depend on null termination, if the file size:

1. is nonzero mod page size, it just works; the remainder of the last
   page reads as zero bytes when mmapped.

2. if an exact multiple of the page size, then instead of directly
   mmapping the file, first mmap a mapping 1 byte (thus 1 page) larger
   with MAP_ANON, then use MAP_FIXED to map the file over top of all
   but the last page. Now the mmapped buffer can safely be used as a C
   string.

If such a solution is acceptable I can try to prepare a patch.

Rich