revert/cherry-pick: work on merge commits as well

Usually you cannot revert a merge because you do not know which
side of the merge should be considered the mainline (iow, what
change to reverse).

With this patch, cherry-pick and revert learn -m (--mainline)
option that lets you specify the parent number (starting from 1)
of the mainline, so that you can:

	git revert -m 1 $merge

to reverse the changes introduced by the $merge commit relative
to its first parent, and:

	git cherry-pick -m 2 $merge

to replay the changes introduced by the $merge commit relative
to its second parent.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-compat-util.h

@@ -381,4 +381,17 @@ static inline int strtoul_ui(char const *s, int base, unsigned int *result)
 	return 0;
 }
 
+static inline int strtol_i(char const *s, int base, int *result)
+{
+	long ul;
+	char *p;
+
+	errno = 0;
+	ul = strtol(s, &p, base);
+	if (errno || *p || p == s || (int) ul != ul)
+		return -1;
+	*result = ul;
+	return 0;
+}
+
 #endif