help.c: Add support for htmldir relative to git_exec_path()

help.c: Add support for htmldir relative to git_exec_path()

If htmldir (in the Makefile) is a relative path, this path will now be
interpreted relative to git_exec_path.  This can be used to create an
installation that can be moved to a different directory without
re-compiling.  The Windows installer (msysgit) is an example for such
a setup.

Note that the Makefile maps htmldir to the define GIT_HTML_PATH.

Signed-off-by: Steffen Prohaska <redacted>
Signed-off-by: Johannes Sixt <redacted>
Signed-off-by: Junio C Hamano <redacted>