tests: git: Avoid ambiguous module import.

Fixes this warning:

WARNING: (guix tests git): `reset' imported from both (git) and (ice-9 control)

This is a followup to commit 07145c8a8c.

* guix/tests/git.scm: Only import LET/EC from (ice-9 control).

guix/tests/git.scm

@@ -23,7 +23,7 @@
   #:use-module (guix utils)
   #:use-module (guix build utils)
   #:use-module (ice-9 match)
-  #:use-module (ice-9 control)
+  #:use-module ((ice-9 control) #:select (let/ec))
   #:export (git-command
             with-temporary-git-repository
             with-git-repository