Merge branch 'jc/am-3-nonstandard-popt' into maint

The code to synthesize the fake ancestor tree used by 3-way merge fallback in "git am" was not prepared to read a patch created with a non-standard -p<num> value. * jc/am-3-nonstandard-popt: test: "am -3" can accept non-standard -p<num> am -3: allow nonstandard -p<num> option