Similar to the previous commit, the functions `write_cruft_pack()` and
`write_filtered_pack()` both compute a "local" variable via the exact
same mechanism:

    const char *scratch;
    int local = skip_prefix(opts->destination, opts->packdir, &scratch);

Not only does this cause us to repeat the same pair of lines, it also
introduces an unnecessary "scratch" variable that is common between both
functions.

Instead of repeating ourselves, let's extract that functionality into a
new function in the repack.h API called "write_pack_opts_is_local()".
That function takes a pointer to a "struct write_pack_opts" (which has
as fields both "destination" and "packdir"), and can encapsulate the
dangling "scratch" field.

Extract that function and make it visible within the repack.h API, and
use it within both `write_cruft_pack()` and `write_filtered_pack()`.
While we're at it, match our modern conventions by returning a "bool"
instead of "int", and use `starts_with()` instead of `skip_prefix()` to
avoid storing the dummy "scratch" variable.

The remaining duplication (that is, that both `write_cruft_pack()` and
`write_filtered_pack()` still both call `write_pack_opts_is_local()`)
will be addressed in the following commit.

Signed-off-by: Taylor Blau <redacted>
---
 builtin/repack.c | 6 ++----
 repack.c         | 5 +++++
 repack.h         | 1 +
 3 files changed, 8 insertions(+), 4 deletions(-)

diff --git a/builtin/repack.c b/builtin/repack.c
index b21799c650..d1449cfe13 100644
--- a/builtin/repack.c
+++ b/builtin/repack.c

@@ -147,8 +147,7 @@ static int write_filtered_pack(const struct write_pack_opts *opts,
 	FILE *in;
 	int ret;
 	const char *caret;
-	const char *scratch;
-	int local = skip_prefix(opts->destination, opts->packdir, &scratch);
+	bool local = write_pack_opts_is_local(opts);
 	const char *pack_prefix = write_pack_opts_pack_prefix(opts);
 
 	prepare_pack_objects(&cmd, opts->po_args, opts->destination);

@@ -232,8 +231,7 @@ static int write_cruft_pack(const struct write_pack_opts *opts,
 	struct string_list_item *item;
 	FILE *in;
 	int ret;
-	const char *scratch;
-	int local = skip_prefix(opts->destination, opts->packdir, &scratch);
+	bool local = write_pack_opts_is_local(opts);
 	const char *pack_prefix = write_pack_opts_pack_prefix(opts);
 
 	prepare_pack_objects(&cmd, opts->po_args, opts->destination);

diff --git a/repack.c b/repack.c
index 19fd1d6d5b..d2ee9f2460 100644
--- a/repack.c
+++ b/repack.c

@@ -77,6 +77,11 @@ const char *write_pack_opts_pack_prefix(const struct write_pack_opts *opts)
 	return pack_prefix;
 }
 
+bool write_pack_opts_is_local(const struct write_pack_opts *opts)
+{
+	return starts_with(opts->destination, opts->packdir);
+}
+
 #define DELETE_PACK 1
 #define RETAIN_PACK 2
 

diff --git a/repack.h b/repack.h
index 5852e2407f..26d1954ae2 100644
--- a/repack.h
+++ b/repack.h

@@ -40,6 +40,7 @@ struct write_pack_opts {
 };
 
 const char *write_pack_opts_pack_prefix(const struct write_pack_opts *opts);
+bool write_pack_opts_is_local(const struct write_pack_opts *opts);
 
 struct repository;
 struct packed_git;

-- 
2.51.0.540.ga7423965ad8