Re: [PATCH 13/20] userdiff tests + docs: document & test "diff.<driver>.x?funcname"

Add the missing documentation for "diff.<driver>.funcname" and test
for how it and "diff.<driver>.xfuncname" interact.
[...]
Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
---

diff --git a/Documentation/config/diff.txt b/Documentation/config/diff.txt

@@ -153,10 +153,22 @@ diff.<driver>.command::
+diff.<driver>.funcname::
 diff.<driver>.xfuncname::
        The regular expression that the diff driver should use to
        recognize the hunk header.  A built-in pattern may also be used.
        See linkgit:gitattributes[5] for details.
++
+When provided as `diff.<driver>.funcname` the regular expression is
+interpreted as a basic regular expression, with

+`diff.<driver>.xfuncname` it's interpreted as an extended regular
+expression.
++
+
+The `*.funcname` and `*.xfuncname` variables behave as if though they
+were one configuration variable for the purposes of what value
+eventually gets used. Setting `*.funcname` will override an earlier
+`*.xfuncname` and vice-versa.