apply: return a specific exit code on failure

Return `GIT_EAPPLYFAIL` on patch application failure so that users can
determine that patch application failed due to a malformed/conflicting
patch by looking at the error code.

include/git2/errors.h

@@ -57,6 +57,7 @@ typedef enum {
 	GIT_RETRY           = -32,	/**< Internal only */
 	GIT_EMISMATCH       = -33,	/**< Hashsum mismatch in object */
 	GIT_EINDEXDIRTY     = -34,	/**< Unsaved changes in the index would be overwritten */
+	GIT_EAPPLYFAIL      = -35,	/**< Patch application failed */
 } git_error_code;
 
 /**

src/apply.c

@@ -24,7 +24,7 @@
 #include "reader.h"
 
 #define apply_err(...) \
-	( giterr_set(GITERR_PATCH, __VA_ARGS__), -1 )
+	( giterr_set(GITERR_PATCH, __VA_ARGS__), GIT_EAPPLYFAIL )
 
 typedef struct {
 	/* The lines that we allocate ourself are allocated out of the pool.