Replace direkt calls to git with $GIT_PROGRAM

Some calls to git ignored the yadm.gpg-program configuration option
and called the first git found in $PATH instead. Make them adhere to
the configured git program by replacing the call with $GIT_PROGRAM.

yadm

@@ -410,13 +410,13 @@ EOF
 
     # operate on the yadm repo's configuration file
     # this is always local to the machine
-    git config --local "$@"
+    $GIT_PROGRAM config --local "$@"
 
     CHANGES_POSSIBLE=1
 
   else
     # operate on the yadm configuration file
-    git config --file="$(mixed_path "$YADM_CONFIG")" "$@"
+    $GIT_PROGRAM config --file="$(mixed_path "$YADM_CONFIG")" "$@"
 
   fi
 
@@ -844,7 +844,7 @@ function set_operating_system() {
 
   case "$OPERATING_SYSTEM" in
     CYGWIN*)
-      git_version=$(git --version 2>/dev/null)
+      git_version=$($GIT_PROGRAM --version 2>/dev/null)
       if [[ "$git_version" =~ windows ]] ; then
           USE_CYGPATH=1
       fi