Add support for git-grep command.

Signed-off-by: Sébastien Gross <seb•ɑƬ•chezwam•ɖɵʈ•org>
grep is only required at compile time.

Signed-off-by: Sébastien Gross <seb•ɑƬ•chezwam•ɖɵʈ•org>
Change --line-number to -n to be compatible with old git versions.

Signed-off-by: Sébastien Gross <seb•ɑƬ•chezwam•ɖɵʈ•org>
fix bug introduced in 361f2d6

implement acons with cons, not nconc
Merge branch 'tarsius/cl-lib' into next
Merge commit '5cddbd6476e510f160e9e2b4f7e4bb3c0a0dba35' into renard/git-grep-support
remove useless (END) marker
Adds an option to determine when to set upstream while pushing.

This answers #562.

This option is useful in the case where the user uses multiple remotes
for a branch.
When pushing changes to the non upstream remote the user is not asked if
he wants to change its upstream.
However, when no upstream is set, the question is asked.
Merge branch 'master' into next

Conflicts:
	magit.el