repos: GitRepo fetch_cmd shouldn't default to "git fetch --all"

The fetch() method for git will call git fetch --all ; this causes
issues when user has multiple remotes. We are able to simply call
'fetch' because by default the origin remote will be used.

From git help fetch:

> When no remote is specified, by default the origin remote will be
used, unless there's an upstream branch configured for the current
branch.

Signed-off-by: Nathan Genetzky <nathan@genetzky.us>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

kas/repos.py

@@ -356,7 +356,7 @@ class GitRepo(RepoImpl):
         return ['git', 'cat-file', '-t', self.refspec]
 
     def fetch_cmd(self):
-        return ['git', 'fetch', '--all']
+        return ['git', 'fetch']
 
     def is_dirty_cmd(self):
         return ['git', 'status', '-s']