builtin/commit.c: memoize git-path for COMMIT_EDITMSG

builtin/commit.c: memoize git-path for COMMIT_EDITMSG

This is a follow up commit for f932729c (memoize common git-path
"constant" files, 10-Aug-2015).

The many function calls to git_path() are replaced by
git_path_commit_editmsg() and which thus eliminates the need to repeatedly
compute the location of "COMMIT_EDITMSG".

Mentored-by: Lars Schneider <redacted>
Mentored-by: Christian Couder <redacted>
Signed-off-by: Pranit Bauva <redacted>
Reviewed-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>