Share git_diff_blobs/git_diff_blob_to_buffer code

This moves the implementation of these two APIs into common code
that will be shared between the two.  Also, this adds tests for
the `git_diff_blob_to_buffer` API.  Lastly, this adds some extra
`const` to a few places that can use it.

2 files changed, 20 insertions, 22 deletions

diff --git a/include/git2/blob.h b/include/git2/blob.h
index 30055b614..93d1c7646 100644
--- a/include/git2/blob.h
+++ b/include/git2/blob.h
@@ -91,7 +91,7 @@ GIT_INLINE(const git_oid *) git_blob_id(const git_blob *blob)
  * @param blob pointer to the blob
  * @return the pointer; NULL if the blob has no contents
  */
-GIT_EXTERN(const void *) git_blob_rawcontent(git_blob *blob);
+GIT_EXTERN(const void *) git_blob_rawcontent(const git_blob *blob);
 
 /**
  * Get the size in bytes of the contents of a blob
@@ -99,7 +99,7 @@ GIT_EXTERN(const void *) git_blob_rawcontent(git_blob *blob);
  * @param blob pointer to the blob
  * @return size on bytes
  */
-GIT_EXTERN(git_off_t) git_blob_rawsize(git_blob *blob);
+GIT_EXTERN(git_off_t) git_blob_rawsize(const git_blob *blob);
 
 /**
  * Read a file from the working folder of a repository
diff --git a/include/git2/diff.h b/include/git2/diff.h
index f1c0cd969..70dbd97aa 100644
--- a/include/git2/diff.h
+++ b/include/git2/diff.h
@@ -802,22 +802,25 @@ GIT_EXTERN(int) git_diff_patch_to_str(
  */
 
 /**
- * Directly run a text diff on two blobs.
+ * Directly run a diff on two blobs.
  *
  * Compared to a file, a blob lacks some contextual information. As such,
- * the `git_diff_file` parameters of the callbacks will be filled
- * accordingly to the following: `mode` will be set to 0, `path` will be set
- * to NULL. When dealing with a NULL blob, `oid` will be set to 0.
+ * the `git_diff_file` given to the callback will have some fake data; i.e.
+ * `mode` will be 0 and `path` will be NULL.
  *
- * When at least one of the blobs being dealt with is binary, the
- * `git_diff_delta` binary attribute will be set to 1 and no call to the
- * hunk_cb nor line_cb will be made.
+ * NULL is allowed for either `old_blob` or `new_blob` and will be treated
+ * as an empty blob, with the `oid` set to NULL in the `git_diff_file` data.
+ *
+ * We do run a binary content check on the two blobs and if either of the
+ * blobs looks like binary data, the `git_diff_delta` binary attribute will
+ * be set to 1 and no call to the hunk_cb nor line_cb will be made (unless
+ * you pass `GIT_DIFF_FORCE_TEXT` of course).
  *
  * @return 0 on success, GIT_EUSER on non-zero callback, or error code
  */
 GIT_EXTERN(int) git_diff_blobs(
-	git_blob *old_blob,
-	git_blob *new_blob,
+	const git_blob *old_blob,
+	const git_blob *new_blob,
 	const git_diff_options *options,
 	git_diff_file_cb file_cb,
 	git_diff_hunk_cb hunk_cb,
@@ -825,22 +828,17 @@ GIT_EXTERN(int) git_diff_blobs(
 	void *payload);
 
 /**
- * Directly run a text diff between a blob and a buffer.
- *
- * Compared to a file, a blob and a buffer lack some contextual information. As such,
- * the `git_diff_file` parameters of the callbacks will be filled
- * accordingly to the following: `mode` will be set to 0, `path` will be set
- * to NULL. When dealing with a NULL blob, `oid` will be set to 0.
+ * Directly run a diff between a blob and a buffer.
  *
- * When at least the blob or the buffer are binary, the
- * `git_diff_delta` binary attribute will be set to 1 and no call to the
- * hunk_cb nor line_cb will be made.
+ * As with `git_diff_blobs`, comparing a blob and buffer lacks some context,
+ * so the `git_diff_file` parameters to the callbacks will be faked a la the
+ * rules for `git_diff_blobs()`.
  *
  * @return 0 on success, GIT_EUSER on non-zero callback, or error code
  */
 GIT_EXTERN(int) git_diff_blob_to_buffer(
-	git_blob *old_blob,
-	char *buffer,
+	const git_blob *old_blob,
+	const char *buffer,
 	size_t buffer_len,
 	const git_diff_options *options,
 	git_diff_file_cb file_cb,