tests: set git config using `git config` for simplicity

I also took the liberty to not set the same config value twice as we
did before.

Differential Revision: https://phab.mercurial-scm.org/D9176

tests/test-convert-git.t

@@ -1,11 +1,7 b''
 #require git
 
-  $ echo "[init]" >> $HOME/.gitconfig
-  $ echo "defaultBranch = master" >> $HOME/.gitconfig
-  $ echo "[core]" >> $HOME/.gitconfig
-  $ echo "autocrlf = false" >> $HOME/.gitconfig
-  $ echo "[core]" >> $HOME/.gitconfig
-  $ echo "autocrlf = false" >> $HOME/.gitconfig
+  $ git config -f $HOME/.gitconfig init.defaultBranch master
+  $ git config -f $HOME/.gitconfig core.autocrlf false
   $ echo "[extensions]" >> $HGRCPATH
   $ echo "convert=" >> $HGRCPATH
   $ cat >> $HGRCPATH <<EOF