Add -v option for git_check_commit.py.

Add -v option for git_check_commit.py.

Doing so, one can see:
$ git gcc-verify a50914d2111c72d2cd5cb8cf474133f4f85a25f6 -v
Checking a50914d2111c72d2cd5cb8cf474133f4f85a25f6: FAILED
ERR: unchanged file mentioned in a ChangeLog: "gcc/common.opt"
ERR: unchanged file mentioned in a ChangeLog (did you mean "gcc/testsuite/g++.dg/pr102955.C"?): "gcc/testsuite/gcc.dg/pr102955.c"
- gcc/testsuite/gcc.dg/pr102955.c
?                ^^             ^

+ gcc/testsuite/g++.dg/pr102955.C
?                ^^             ^

contrib/ChangeLog:

	* gcc-changelog/git_check_commit.py: Add -v option.
	* gcc-changelog/git_commit.py: Print verbose diff for wrong
	filename.