grep: better support for binary files

This series improves support of git grep for binary files. It tries to pick the low hanging fruits; at the end you can search _in_ files that contain NUL characters, but you can't search _for_ NULs, yet.

[PATCH 1/7] grep: add test script for binary file handling

This patch adds a simple test script documenting what git grep can do with binary files.

[PATCH 2/7] grep: refactor handling of binary mode options [PATCH 3/7] grep: --count over binary

These two makes git grep handle counting in binary files like GNU grep does.

[PATCH 4/7] grep: use memmem() for fixed string search
[PATCH 5/7] grep: continue case insensitive fixed string search after NUL chars

These two patches make git grep -F work on binary files.

[PATCH 6/7] grep: add regmatch(), a wrapper for REG_STARTEND handling [PATCH 7/7] grep: use regmatch() for line matching

The final patches make git grep work on binary files if the platform's regexec() supports the flag REG_STARTEND. Our own version in compat/ doesn't, unfortunately.

 grep.c                 |   70 ++++++++++++++++++++++++++++-------------------
 t/t7008-grep-binary.sh |   54 +++++++++++++++++++++++++++++++++++++
 2 files changed, 96 insertions(+), 28 deletions(-)