Implement gsource/gvault/gfunction/gcdefine in commands/source.pl

These are versions of source/vault/function/cdefine commands that link file in
the source tree on a webserver's git browser instead of sending the data to
pastebin.  So a query like !gfunction bad_attack will simply return a URL like:

"${git_browser_url}?p=crawl.git;a=blob;f=crawl-ref/source/misc.cc;hb=HEAD#l2199"

where $git_browser_url is set in src/Helper.pm currently.  The link takes the
browser to the right source file and line number in the git browser.  All of the
usual kinds of queries work, but any ending line number in the query is ignored
since the git browser can't use this information.