misc/git: Fix non-literal period in pre-commit grep

Looks like this was intended to match a literal period to restrict
this to `.go` files, but in POSIX grep, the unescaped period matches
any character.

Change-Id: I20e00323baa9e9631792eff5035966297665bbee
Reviewed-on: https://go-review.googlesource.com/19880Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/git/pre-commit

@@ -10,7 +10,7 @@
 #
 # This script does not handle file names that contain spaces.
 
-gofiles=$(git diff --cached --name-only --diff-filter=ACM | grep '.go$')
+gofiles=$(git diff --cached --name-only --diff-filter=ACM | grep '\.go$')
 [ -z "$gofiles" ] && exit 0
 
 unformatted=$(gofmt -l $gofiles)