Initial implementation of git save format

This saves the dive data into a git object repository instead of a
single XML file.

We create a git object tree with each dive as a separate file,
hierarchically by trip and date.

NOTE 1: This largely duplicates the XML saving code, because trying to
share it seemed just too painful: the logic is very similar, but the
details of the actual strings end up differing sufficiently that there
are tons of trivial differences.

The git save format is line-based with minimal quoting, while XML quotes
everything with either "<..\>" or using single quotes around attributes.

NOTE 2: You currently need a dummy "file" to save to, which points to
the real save location: the git repository and branch to be used.  We
should make this a config thing, but for testing, do something like
this:

	echo git /home/torvalds/scuba:linus > git-test

to create that git information file, and when you use "Save To" and
specify "git-test" as the file to save to, subsurface will use the new
git save logic to save to the branch "linus" in the repository found at
"/home/torvalds/scuba".

NOTE 3: The git save format uses just the git object directory, it does
*not* check out the result in any git working tree or index.  So after
you do a save, you can do

     git log -p linus

to see what actually happened in that branch, but it will not affect any
actual checked-out state in the repository.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>

subsurface-configure.pri

@@ -126,7 +126,7 @@ LIBS *= $$XSLT_LIBS $$XML2_LIBS
 # We're searching for:
 #  libzip
 #  sqlite3
-link_pkgconfig: PKGCONFIG += libzip sqlite3
+link_pkgconfig: PKGCONFIG += libzip sqlite3 libgit2
 
 # Add libiconv if needed
 link_pkgconfig: packagesExist(libiconv): PKGCONFIG += libiconv