repository: do not interpret all files as gitlinks in discovery

When trying to find a discovery, we walk up the directory
structure checking if there is a ".git" file or directory and, if
so, check its validity. But in the case that we've got a ".git"
file, we do not want to unconditionally assume that the file is
in fact a ".git" file and treat it as such, as we would error out
if it is not.

Fix the issue by only treating a file as a gitlink file if it
ends with "/.git". This allows users of the function to discover
a repository by handing in any path contained inside of a git
repository.

src/repository.c

@@ -410,7 +410,7 @@ static int find_repo(
 					break;
 				}
 			}
-			else if (S_ISREG(st.st_mode)) {
+			else if (S_ISREG(st.st_mode) && git__suffixcmp(path.ptr, "/" DOT_GIT) == 0) {
 				error = read_gitfile(&repo_link, path.ptr);
 				if (error < 0)
 					break;

tests/repo/discover.c

@@ -189,3 +189,13 @@ void test_repo_discover__ceiling_should_not_affect_gitdir_redirection(void)
 	ensure_repository_discover(REPOSITORY_ALTERNATE_FOLDER_SUB_SUB, ceiling_dirs.ptr, SUB_REPOSITORY_GITDIR);
 	ensure_repository_discover(REPOSITORY_ALTERNATE_FOLDER_SUB_SUB_SUB, ceiling_dirs.ptr, DISCOVER_FOLDER);
 }
+
+void test_repo_discover__discovery_starting_at_file_succeeds(void)
+{
+	int fd;
+
+	cl_assert((fd = p_creat(SUB_REPOSITORY_FOLDER "/file", 0600)) >= 0);
+	cl_assert(p_close(fd) == 0);
+
+	ensure_repository_discover(SUB_REPOSITORY_FOLDER "/file", ceiling_dirs.ptr, SUB_REPOSITORY_GITDIR);
+}