Add git configuration templates to improve diff hunk header detection.

This is a follow-up to commit 9fc8ae4171, which
missed two hunks from <https://issues.guix.gnu.org/50363>.

Reported by hackeryarn on #guix.

* .gitattributes, etc/git/gitconfig: New files.

Signed-off-by: Marius Bakke <marius@gnu.org>

.gitattributes

@@ -0,0 +1,3 @@
+*.scm       diff=scheme
+*.scm.in    diff=scheme
+*.texi      diff=texinfo

etc/git/gitconfig

@@ -0,0 +1,5 @@
+[diff "scheme"]
+	xfuncname = "^(\\(define.*)$"
+
+[diff "texinfo"]
+	xfuncname = "^@node[[:space:]]+([^,]+).*$"