rebase: use strvec_pushf() for format-patch revisions

rebase: use strvec_pushf() for format-patch revisions

In run_am(), a strbuf is used to create a revision argument that is then
added to the argument list for git format-patch using strvec_push().
Use strvec_pushf() to add it directly instead, simplifying the code
and plugging a small leak on the error code path.

Signed-off-by: René Scharfe <redacted>
Signed-off-by: Junio C Hamano <redacted>