Implement global/system file search paths

The goal of this work is to expose the search logic for "global",
"system", and "xdg" files through the git_libgit2_opts() interface.

Behind the scenes, I changed the logic for finding files to have a
notion of a git_strarray that represents a search path and to store
a separate search path for each of the three tiers of config file.
For each tier, I implemented a function to initialize it to default
values (generally based on environment variables), and then general
interfaces to get it, set it, reset it, and prepend new directories
to it.

Next, I exposed these interfaces through the git_libgit2_opts
interface, reusing the GIT_CONFIG_LEVEL_SYSTEM, etc., constants
for the user to control which search path they were modifying.
There are alternative designs for the opts interface / argument
ordering, so I'm putting this phase out for discussion.

Additionally, I ended up doing a little bit of clean up regarding
attr.h and attr_file.h, adding a new attrcache.h so the other two
files wouldn't have to be included in so many places.

1 files changed, 4 insertions, 3 deletions

diff --git a/src/config.h b/src/config.h
index db5ebb3b7..c43e47e82 100644
--- a/src/config.h
+++ b/src/config.h
@@ -12,10 +12,11 @@
 #include "vector.h"
 #include "repository.h"
 
-#define GIT_CONFIG_FILENAME ".gitconfig"
-#define GIT_CONFIG_FILENAME_ALT ".config/git/config"
-#define GIT_CONFIG_FILENAME_INREPO "config"
 #define GIT_CONFIG_FILENAME_SYSTEM "gitconfig"
+#define GIT_CONFIG_FILENAME_GLOBAL ".gitconfig"
+#define GIT_CONFIG_FILENAME_XDG    "config"
+
+#define GIT_CONFIG_FILENAME_INREPO "config"
 #define GIT_CONFIG_FILE_MODE 0666
 
 struct git_config {