internal/git: take diff stat from merge base

The default behavior for `git diff --stat <target>` is to include all
the difference between HEAD and <target> (a diff union), however, we
want only what was changed from <target> to HEAD, regardless how far
<target> already is from HEAD. For this, we need to consider <target> as
our merge base, and for that, we could add `--merge-base` to the
command.

However, the --merge-base option is available only in recent versions of
Git (v2.30+), which are still not installed everywhere, so for that
we're going to use the 'A...B' notation, that for `git diff` means:

    git diff $(git merge-base <taget> HEAD) HEAD

Signed-off-by: Bruno Meneguele <[email protected]>

internal/git/git.go

@@ -132,7 +132,7 @@ func Log(sha1, sha2 string) (string, error) {
 		return "", errors.Errorf("Can't load git log %s..%s", sha1, sha2)
 	}
 
-	diffCmd := New("diff", "--stat", sha1)
+	diffCmd := New("diff", "--stat", fmt.Sprintf("%s...", sha1))
 	diffCmd.Stdout = nil
 	diffOutput, err := diffCmd.Output()
 	if err != nil {