Allow for creating patch series

This allows for specifying a revision range using the id2 parameter of
/patch/. The output that is produced is similar to

    $ git format-patch --stdout id2..id

Signed-off-by: Lukas Fleischer <cgit@cryptocrack.de>

3 files changed, 20 insertions, 7 deletions

diff --git a/cmd.c b/cmd.c
index 9893710..0202917 100644
--- a/cmd.c
+++ b/cmd.c
@@ -98,7 +98,7 @@ static void repolist_fn(struct cgit_context *ctx)
 
 static void patch_fn(struct cgit_context *ctx)
 {
-        cgit_print_patch(ctx->qry.sha1, ctx->qry.path);
+        cgit_print_patch(ctx->qry.sha1, ctx->qry.sha2, ctx->qry.path);
 }
 
 static void plain_fn(struct cgit_context *ctx)
diff --git a/ui-patch.c b/ui-patch.c
index 72c9749..bb39294 100644
--- a/ui-patch.c
+++ b/ui-patch.c
@@ -11,7 +11,7 @@
 #include "html.h"
 #include "ui-shared.h"
 
-void cgit_print_patch(char *hex, const char *prefix)
+void cgit_print_patch(char *hex, const char *old_rev, const char *prefix)
 {
         struct rev_info rev;
         struct commit *commit;
@@ -33,16 +33,29 @@ void cgit_print_patch(char *hex, const char *prefix)
                 return;
         }
 
-        if (commit->parents && commit->parents->item) {
+        if (old_rev) {
+                if (get_sha1(old_rev, old_sha1)) {
+                        cgit_print_error("Bad object id: %s", old_rev);
+                        return;
+                }
+                if (!lookup_commit_reference(old_sha1)) {
+                        cgit_print_error("Bad commit reference: %s", old_rev);
+                        return;
+                }
+        } else if (commit->parents && commit->parents->item) {
                 hashcpy(old_sha1, commit->parents->item->object.sha1);
-                sprintf(rev_range, "%s..%s", sha1_to_hex(old_sha1),
-                        sha1_to_hex(sha1));
         } else {
                 hashclr(old_sha1);
+        }
+
+        if (is_null_sha1(old_sha1)) {
                 memcpy(rev_range, sha1_to_hex(sha1), 41);
+        } else {
+                sprintf(rev_range, "%s..%s", sha1_to_hex(old_sha1),
+                        sha1_to_hex(sha1));
         }
 
-        patchname = fmt("%s.patch", sha1_to_hex(sha1));
+        patchname = fmt("%s.patch", rev_range);
         ctx.page.mimetype = "text/plain";
         ctx.page.filename = patchname;
         cgit_print_http_headers(&ctx);
diff --git a/ui-patch.h b/ui-patch.h
index 1641cea..acd37fd 100644
--- a/ui-patch.h
+++ b/ui-patch.h
@@ -1,6 +1,6 @@
 #ifndef UI_PATCH_H
 #define UI_PATCH_H
 
-extern void cgit_print_patch(char *hex, const char *prefix);
+extern void cgit_print_patch(char *hex, const char *old_rev, const char *prefix);
 
 #endif /* UI_PATCH_H */