[PATCH] Add an option to specify a file to config builtin

There are (really!) systems where using environment variables is very
cumbersome (yes, Windows, it has problems unsetting them). Besides this
form is shorter.

Signed-off-by: Alex Riesen <redacted>
---
 Documentation/git-config.txt |   29 ++++++++++++++++-------------
 builtin-config.c             |    9 ++++++++-
 2 files changed, 24 insertions(+), 14 deletions(-)