HTTP cookies: do tilde expansion on http.cookieFile

HTTP cookies: do tilde expansion on http.cookieFile

Git config http.cookieFile must have ~ expansion, compare [1].

It also should be an absolute path. While a relative path is allowed,
C git just passes the value on to libcurl, so it'll be relative to the
current working directory and thus not work in all directories.

Log a warning if the path is relative.

(Alternatives would be to throw an exception, or to resolve the path
relative to the .git directory, or relative to the working tree root,
or relative to the config file it occurs in. But C git does not seem
to do either.)

[1] https://github.com/git/git/commit/e5a39ad8e

Bug: 571798
Change-Id: I5cdab6061d0613ac7d8cb7977e5b97f5b88f562d
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>