[PATCH v3] format-patch --signature-file <file>

v3 patch to add format-patch --signature-file <file> option.

This revision includes changes suggested by Jeff King:

  - Instead of a custom OPTION_CALLBACK, use OPT_FILENAME which
	correctly resolves file names which are not evaluated by the
	shell such as "--signature-file=<file>"

  - Use strbuf_read_file() which eliminates the arbitrary size limit
	and simplifies the code.

  - Generate an error if the file cannot be read instead of quietly
	continuing.  This is accomplished with strbuf_read_file().

  - Die if both --signature and --signature-file are used.

Jeremiah Mahler (1):
  format-patch --signature-file <file>

 Documentation/git-format-patch.txt |  4 ++++
 builtin/log.c                      | 13 +++++++++++++
 t/t4014-format-patch.sh            | 28 ++++++++++++++++++++++++++++
 3 files changed, 45 insertions(+)