Tighten git-gc and git-nw-repack patterns to avoid false positives
Anchor git-gc proc matches to start of line, further tightening

Additional enhancements to reduce the likelihood of false positive
matches causing backups to fail after the GC cooldown window is
exceeded.

These should be much more resilient to matching either "gc" or
"nw-repack" in other commands. The changes here introduce the
following new rules for matching gc / repack processes::w

 - Only match "git" at the beginning of the line. Verified all
   GC / nw-repack invocations follow this pattern.
 - Match either "git gc" or "git --opts ... gc" but require a space
   before the subcommand (gc / nw-repack) in either case. i.e. Don't
   match "git somethingc".
 - Match "git ... gc" where "gc" is anchored at end of line.
 - Match "git gc --opts" where gc is followed by a space.
Merge pull request #24 from github/fix-gc-patterns

Tighten up git gc / git nw-repack matching patterns